The present invention relates to a process for the cryogenic distillation of an air feed. As used herein, the term "air feed" generally means atmospheric air but also includes any gas mixture containing at least nitrogen and oxygen.
There is a need in the chemicals industry for oxygen-rich gas at elevated pressure and at varying purity, ranging from moderate purity gas comprised of 97 mole % or less of oxygen to high purity gas comprised of 97 mole % or greater of oxygen, preferably 99 mole % or greater of oxygen. Some applications may also require oxygen at two different purities and at elevated pressure.
The objective of the present invention is a cryogenic process that produces at least two different oxygen-rich gaseous products at differing purities and that produces at least two of these oxygen-rich gaseous products from a mixing column system, optionally at elevated pressure if desired.
U.S. Pat. Nos. 5,315,833 and 5,396,773 assigned to Liquid Air Engineering Corp. describe a double column air separation process in which two oxygen-rich products of differing purities are produced from a double column assembly. Multiple reboilers are included in the lower pressure column of the assembly to permit more efficient production of two products. This process does not use a mixing column to produce elevated pressure products and so requires feed air to the process to be compressed to a high pressure (the pumped liquid oxygen arrangement). This negatively impacts the energy consumption of the process.
European Patent Application Number 96301423.8 assigned to Air Products and Chemicals, Inc. describes a triple column process in which a double column apparatus is supplemented with a third column to permit the production of a second oxygen-rich product of high purity. This process does not employ a mixing column and so requires feed air to be compressed to a high pressure (the pumped liquid oxygen arrangement), negatively impacting the energy consumption of the process.
U.S. Pat. Nos. 4,022,030 and 5,291,737 assigned to L'Air Liquide and European Patent Application Number 94302954.6 assigned to The BOC Group plc describe processes in which a mixing column is used to produce a single oxygen-rich gaseous product. Although these L'Air Liquide patents describe a process by which multiple purity oxygen-rich products could be produced, only one of the products is produced directly from a mixing column. Likewise, the BOC patent application teaches a mixing column process for the production of a single oxygen-rich gaseous product only. These processes all teach that liquid oxygen be supplied to the mixing column as a single purity stream.
U.S. Pat. No. 5,454,227, assigned to The BOC Group, describes a process in which a single oxygen-rich gaseous product is produced from a mixing column. This process uses a turboexpanded air stream to supply vapor to the bottom of the mixing column, and would typically be more suitable for production of lower pressure oxygen-rich products than the processes of U.S. Pat. Nos. 4,022,030 and 5,291,737. U.S. Pat. No. 5,454,227 also teaches the production of a single oxygen-rich gaseous product through the use of a mixing column that is supplied with a single purity stream of liquid oxygen.
U.S. Pat. No. 5,704,228 assigned to L'Air Liquide teaches the use of a mixing column having a reboiler in its sump but does not teach the production of dual purity oxygen-rich products, nor does it teach the withdrawal of dual purity oxygen-rich liquid streams from the distillation column system.